1. Field of Invention
This invention relates to the processing of electronic, optical and other materials and, more particularly, to the cleaning or chemical conditioning of semiconductor wafer surfaces and to the etching or reactive modification of semiconductor surface layers.
2. Description of the Prior Art
The current state of the art in ULSI chip fabrication involves the repeated deposition or growth, patterning, and etching of thin films of semiconductor, metal, and dielectric materials. Between these process steps are typically thirty or more steps which involve cleaning of surface contaminants and residues, removal of damaged surface layers, and/or chemical conditioning of the material surface to optimize subsequent processing. Traditional wafer cleaning and conditioning steps involve wet chemical processes such as the "RCA standard clean" and a variety of HF acid dips or sprays, followed by ultra-pure water rinses. These processes facilitate the removal of organic contaminants and trace metals through selective chemical reactions, as well as the removal (etching) of contaminated and damaged substrate layers to expose a pristine surface for subsequent processing. However, the use of wet chemical processes in semiconductor fabrication is increasingly undesirable due to the expense and hazards of wet chemical handling, surface tension limitations to liquid permeation into submicron features, and the high level of metallic and particulate contamination in the wet process chemicals.
The industry trend in semiconductor manufacture is toward the clustering of sequential single wafer processes in which atmospheric exposure of the substrate wafers is limited by introducing gas-phase or "dry" wafer cleaning and conditioning processes. To function as successful replacements for traditional wet cleaning processes, these "dry" cleaning processes must facilitate the removal of organic, metallic and other contaminants, and allow "light" (non-aggressive) etching of damaged and contaminated surface layers via gas-phase reactions with the substrate material. In addition to cleaning and conditioning planar surfaces, these processes must allow cleaning of high aspect ratio features in the substrate material such as deep trenches and vias, where contaminated layers may exist on the bottoms and/or sidewalls.
It is an object of the present invention to allow the gas-phase cleaning, conditioning, and damage-free removal of contaminated layers with such directional facility.
Etching, or subtractive processing of a patterned film, is an example of a previously "wet" chemical process step in semiconductor manufacturing which is now commonly performed in the gas-phase through exposure of the wafer surface to a glow discharge or plasma of a reactive gas. It has been established that gas-phase free radicals such as F, Cl, and Br allow the etching of Si, SiO.sub.2 and other semiconductor materials (for instance Cl will etch GaAs and InP) through the formation of volatile substrate products. These radical species are generated in large concentrations in plasma or dry etching processes, and the interaction between chemisorbed free radicals and energetic ions from the plasma results in rapid directional etching. There is significant interest in the use of gas plasmas and plasma products in wafer cleaning processes as well. Ruzyllo, J., "Evaluating the Feasibility of Dry Cleaning of Silicon Wafers" Microcontamination, pp. 39-43 March 1988. However, plasma etching and cleaning processes suffer from the shortcoming that thin films, such as SiO.sub.2 gate dielectric layers, can be damaged by the high energy particle bombardment, the ultraviolet (UV) radiation, and/or the electrical nonuniformities generated in the plasma environment. To avoid structural damage of such thin films, etchant particle energies must be reduced below the material lattice displacement energies, which are typically on the order of 10 eV. Recent developments in high density plasma reactors, which use inductive coupling or electron cyclotron resonance (ECR) excitation, offer reduced plasma particle bombardment energies, but may still cause electrical film damage due to the electrical coupling between the etching substrate features and the plasma. Consequently, alternative techniques for the directional etching and cleaning of electronic materials which occur in an electrically neutral gas-phase environment are needed.
It has been shown that free radicals alone (e.g., F, Cl, H) will etch some semiconductor materials at usable rates, albeit much slower than with the ion-enhanced etching effects of the plasma. Coburn, J., "Dual Atom Beam Studies of Etching and Related Surface Chemistries", Pure and Appl. Chem., Vol. 64, No. 5, pp. 709-713, 1992; and Geis, M., Efremow, N., Pang, S. and Anderson, A , "Hot-jet etching of Pb, GaAs, and Si", J.Vac.Sci.Technol.B, Vol 5, No. 1, January/February, 1987. It has also been contended that vibrationally "hot" or excited molecular species (e.g., SF.sub.6 *, Cl.sub.2 *) also spontaneously etch some semiconductor materials so that full thermal dissociation of the parent gases into free radicals is not required to achieve etching. Saito, Y., Yamaoka, O. and Yoshida, A., "Plasmaless Etching of Silicon Using Chlorine Trifluoride", J.Vac.Sci.Technol.B, Vol. 9, No. 5, pp. 2503-2506, September/October, 1991; Chuang, T., "Multiple Photon Excited SF.sub.6 Interaction with Silicon Surfaces", J. Chem. Phys., Vol. 74, No. 2, pp. 1453-1460, 1981; Suzuki, K., Hiraoka, S. and Nishimatsu, S., "Anisotropic Etching of Polycrystalline Silicon with a Hot Cl.sub.2 Molecular Beam", J.Appl.Phys., Vol. 64, No. 7, pp. 3697-3705, 1988; and Suzuki, K., Ninomiya, K., Nishimatsu, S. and Okada, O., "Si Etching with a Hot SF.sub.6 Beam and the Etching Mechanism", Jpn.J.Appl.Phys., 26, pp. 166-173, 1987. However, these species are difficult to generate in large quantities and with good uniformity such that the chemistry may be exploited on the scale of a full-sized wafer, currently 150 mm to 200 mm in diameter. Attempts to generate large quantities of such reactive species outside of the plasma environment have included plasma "downstream" reactors, in which the reactive species are transported from a plasma chamber to a secondary chamber where the material is isolated from the harmful radiative effects of the plasma.
Beams of free radicals and vibrationally hot molecules have been generated in the research environment to investigate their utility as damage-free etchants for electronic materials, where substrate lattice damage is avoided since the particle energies are typically below 10 eV. Vasile, M. and Stevie, F., "Reaction of Atomic Fluorine with Silicon:
The Gas Phase Products", J.Appl.Phys., Vol. 53, No. 5, pp. 3799-3805, May, 1982; U.S. Pat. No. 4,734,152 to Geis et al.; Geis et al. article, supra; and Suzuki et al. article, supra. The use of free radical beams has the advantage that etchant species are delivered to the substrate in a directional manner such that anisotropic or directional etching can be realized. In addition, the beams are transported in a vacuum chamber without substantial gas-phase collisions such that collisional deactivation of the thermally or otherwise activated etchant species does not occur. However, these laboratory scale studies commonly utilize narrowly directed beams created through the use of small capillary tubes or microchannel arrays which provide acceptable etching uniformity over very small substrate areas.
Efforts have been made to introduce directional etching or deposition processes in commercial semiconductor fabrication by attempting to scale-up the laboratory tested molecular beam technology. However, none of these efforts have achieved widespread use in the semiconductor industry which requires good process uniformity over substrate diameters as large as 200 mm, while attaining acceptable wafer throughput. U.S. Pat. No. 4,774,416 to Askary et al. describes a method for employing effusive molecular beam technology for deposition or etching over large substrate areas through the use of a large microchannel plate with thousands of densely packed microscopic channels (100-10,000 per square centimeter) with individual diameters on the order of 10 .mu.m. The design of laboratory-scale molecular beam sources, particularly those where the gas mean free path .lambda. is larger than both the microchannel diameter and length (.lambda.A&gt;&gt;a, .lambda..gtoreq.L), has been well documented in the literature. Jones, R., Olander, D. and Kruger, V., "Molecular-Beam Sources Fabricated from Multichannel Arrays. I. Angular Distributions and Peaking Factors", J.Appl.Phys., Vol 40, No 11, pp. 4641-4649, 1969; Glines, A., Carter, R. and Anton, A., "An alternative for gas dosing in ultrahigh vacuum adsorption studies", Rev.Sci.Instrum , Vol. 63, No. 2, pp. 1826-1833, February, 1992. The Askary et al. process involves a scale-up of the earlier laboratory experiments carried out by Jones et al. in 1969. In this gas flow regime, which is often referred to as "transparent", a large percentage of the gas species which flow from a gas reservoir through a microchannel to form the molecular beam do so without gas-to-wall or gas-to-gas interactions inside a microchannel. Therefore, to efficiently produce thermally activated etchant species, the source gas must be thermally or otherwise activated in a furnace or plasma chamber preceding the microarray, or by use of laser beam excitation after exiting the array. However, it is currently not possible to manufacture densely packed microchannel plates in high purity, inert ceramic materials which resist chemical attack by the wide variety of halogen species which are useful in substrate etching and surface cleaning processes. This type of effusive molecular beam technology has found its greatest use in the field of molecular beam epitaxy for the directed delivery of room temperature deposition precursor gases.
The molecular beam source design described in U.S. Pat. No. 5,188,671 to Zinck et al. is an example of the application of the Askary et al. technology for the delivery of deposition precursors. U.S. Pat. No. 4,540,466 to Nishizawa describes a process in which gas is activated by UV light in a high pressure reservoir and flows through an injector plate or gas "shower head" which under some conditions may preferentially orient the gas flow. U.S. Pat. No. 4,886,571 to Suzuki et al. describes a related process by which gas may be thermally activated by contact with an "activation surface" and subsequently directed through the use of a cooled collimator plate to quench the non-directed active gas particles. The operational flow regime and design criteria for the collimator plate are not defined so the extent to which molecular beams are utilized is not explained. Suzuki et al., in U.S. Pat. Nos. 4,901,667 and 5,108,543, further expand on this process and device by defining the gas "activation surface" as a furnace from which the gas flows effusively through a thin aperture plate which under some conditions will act like a cosine emitter. Again, a secondary cooled collimating plate is described for the purpose of deactivating non-directed gas species to improve the directionality of etching. However, the secondary collimating plate reduces gas usage efficiency and, over a wide range of operating conditions, contributes to problems of etching uniformity over large areas. The processes of Askary et al. and Suzuki et al. both suffer the limitations of an effusive beam technology which require high and uniform activated or dissociated particle concentrations to be present in the high pressure furnace or "activating" chamber, as is discussed at length in several proposed furnace designs by Suzuki et al. in U.S. Pat. No. 4,901,667.
It is, therefore, a further object of the present invention to overcome these limitations by achieving activation and formation of the gas beams in the same element, through appropriate usage of the hydrodynamics of collisionally opaque molecular beams where the gas mean free path .lambda. is smaller than the dimensions of the beam forming channel over a significant portion of its length. See Gray, D. and Sawin, H., "Design considerations for high-flux collisionally opaque molecular beams", J.Vac.Sci.Technol.A, Vol. 10, No. 5, pp. 3229-3238, 1992; Giordmaine, J. and Wang, T., "Molecular Beam Formation By Long Parallel Tubes*", J.Appl.Phys., Vol. 31, No. 3, pp. 463-471, 1960; and Murphy, D., "Wall collisions, angular flux, and pumping requirements in molecular flow through tubes and microchannel arrays", J.Vac.Sci.Technol.A, Vol. 7, No. 5, pp. 3075-3091, September/October, 1989.
Nearly all of the earlier work regarding the use of free radical and excited molecular beams in semiconductor manufacture has focused on development of a directional etching process for pattern transfer into thin films through a non-erodible mask. However, the substrate etching rates observed in these processes are often too slow to offer a realistic wafer processing time, and significant mask undercut is often observed. To achieve acceptable manufacturing throughputs, a single wafer etching process must typically achieve 0.1 to 1 micron/min. etching rates with a very high degree of anisotropy (directionality) in pattern transfer. The device shown in U.S. Pat. Nos. 4,901,667 and 5,108,543 to Suzuki et al., for instance, cannot achieve SF.sub.6 etching rates of silicon over 500 nm/min. in the regime in which molecular beam formation could occur at realistic pumping speeds. U.S. Pat. No. 5,108,778 to Suzuki et al. describes the use of supersonic nozzles and "seeding" techniques to increase the active particle beam energies above 1 eV to increase etching rates, but this process is not practically scalable to allow etching of large areas with good uniformity. Finally, U.S. Pat. No. 5,110,407 to Ono et al. describes the necessity of using sidewall passivant species such as oxygen or nitrogen radicals during molecular beam etching to prevent the undercut of mask patterns. However, this process is not commensurate with the uniform processing of semiconductor wafers as large as 200 mm. All of these complications and inefficiencies have precluded the use of the previously described molecular beam technologies in the semiconductor wafer manufacturing environment.
It is another object of the present invention to realize an efficient process for the uniform delivery of a directed, thermally activated reactive gas to a large area substrate for material surface cleaning, material removal, or reactive modification in a manufacturing process tool, while overcoming many of the limitations of the above described effusive molecular beam technologies.